The invention has application to the control of fluid flow and particularly the control of fluids containing abrasive material which can cause undesirable wear and erosion of control valve parts.
The use of abrasive materials in a fluid jet is well known, for example for machining operations such as cutting, drilling and surface finishing. In one known arrangement, a high pressure carrier fluid, for example water or air, is pumped into a vessel containing abrasive particles to force a mixture of the abrasive particles entrained in the carrier fluid as a slurry through a nozzle which forms a well-defined abrasive jet.
In a known arrangement a vessel is charged with an abrasive particulate material. A pressurised supply of carrier fluid is applied to the vessel where the carrier fluid and abrasive material mix to form an abrasive slurry which is discharged through the jet-forming nozzle. However, the duration of the supply of the abrasive slurry is limited by the capacity of the vessel and cannot continue without an interruption while the vessel is being refilled with abrasive material.
To maintain as near as possible a continuous abrasive flow from the jet nozzle, it is known to provide a pair of vessels in parallel or series so that one vessel can be de-pressurised and re-charged with abrasive while the other vessel, being pressurised with a carrier fluid, provides the abrasive supply to the nozzle.
European patent application No. 313,700 (Krasnoff) describes a water jet cutting system using an abrasive slurry. The inlets and outlets of a pair of vessels are connected to a pair of changeover valves. The valves are coupled together to alternately connect the vessels for recharging by induction of a slurry from a reservoir, then for pressurised discharge of the slurry at a nozzle.
International patent application No. WO 90/15694 (Saunders et al) describes a device for delivering abrasive liquid slurry to a cutting jet. There is a pair of pressure vessels connected in parallel between a supply of pressurised water and a nozzle. One vessel supplies an abrasive mixture under high pressure to the nozzle while the other vessel is replenished with abrasive from a common hopper.
A clean carrier liquid (water) is fed at a high pressure via an inlet valve into the top of the first vessel containing the abrasive mixture to force abrasive slurry from the vessel and via an outlet valve to the jet nozzle. The inlet valve is closed to stop delivery of the abrasive from that vessel. A flushing valve is opened to flush the outlet valve with a clean liquid flow allowing the outlet valve to be closed without abrasive particles causing undue wear of the moving parts. The flushing causes variations in the abrasive concentration of the abrasive mixture which are smoothed by a vessel downstream of the outlet valve.
U.S. Pat. No. 5,800,246 (Naoyoshi Tomioka) describes a blasting apparatus having a pair of vessels in parallel between a nozzle and a common hopper holding an abrasive material. Each vessel alternately recharges with abrasive from the hopper and then discharges the abrasive via the nozzle.
Wherever valves are used to control the flow of abrasive material, the valve is vulnerable to rapid wear and deterioration of the valve seal surfaces. In the absence of special techniques to avoid this deterioration, valves used to control abrasive flows can be expected to have a short operating lifespan. Many valves are unsuitable for use in the presence of a high-pressure abrasive flow.